Runway 12/30
In the transcript, both Runway 12 and Runway 30 are mentioned. Both are actually the same runway; the difference lies in which direction a plane is facing as it travels along the runway. The KLM plane, for example, taxis along Runway 12 (short for 120°, the direction it is then facing), then turns 180° and attempts takeoff in the opposite direction on what for it has then become Runway 30 (short for 300°).

1658:14.8
The transcript uses standard aviation notation to represent time. 1658:14.8 signifies 4:58 p.m. and 14.8 seconds.

KLM 4805
This aircraft is a Boeing 747 carrying 234 passengers and 14 crew. It is a chartered flight bound from Amsterdam to Las Palmas in the Canary Islands. Like the other airplane involved in the crash, it is diverted to Tenerife's airport when the Las Palmas airport suddenly closes. All those onboard this plane when it crashes die. (One passenger, a tour guide, got off the plane in Tenerife.)

backtrack
Backtracking means using the same runway for both taxiing and takeoff, i.e., taxiing down it in one direction then turning around and taking off in the opposite direction.

12 for takeoff Runway 30
See Runway 12/30

holding position
As Los Rodeos Airport fills up on this day with an uncommon number of large aircraft, Air Traffic Control orders the planes to line up at the end of the landing runway (see number 2 on the diagram).

Charlie 1
The transverse taxiways are identified as Charlie 1, 2, 3, and 4, as shown in the diagram.

Clipper 1736
Pan Am 1736 carries 378 passengers, 16 crew, and two Pan Am employees, who boarded in Tenerife and are sitting in the cockpit jumpseats. It departed from Los Angeles bound for the same intended destination as KLM 4805—Las Palmas's Gran Canaria International Airport—but was diverted to Tenerife. Sixty-one people on this plane will survive the impending collision, though nine of these later die from their injuries.

takeoff minimums
Every airport has its own minimum visibility requirement for takeoffs. Here, the Pan Am captain, noticing the thickening fog, surmises that the poor visibility will not meet the takeoff minimums and therefore no planes will be cleared to take off until the visibility improves.

They put a bomb (in) the terminal
At Gran Canaria International Airport, the intended destination for both aircraft, Canary Island separatists exploded a bomb in the concourse earlier that day, injuring several people. They also called in a threat of a second bomb. Gran Canaria shut down for several hours, causing the temporary diversion of KLM 4805 and Pan Am 1736 to Los Rodeos Airport on Tenerife.

90-degree turn
The Pan Am copilot has a small map of the airport, which shows that the first taxiway is a 90-degree turn. But as the plane taxis down the runway, the flight crew apparently can't decide whether Air Traffic Control considers this a taxiway proper.

it's a 45
Still looking at his map, the Pan Am copilot is probably referring to either Charlie 2 or Charlie 3 taxiways here, both of which lie at roughly 45° angles to the runway. For them, however, they would each represent roughly 135° turns, hence their eventual decision to turn on Charlie 4, which is a much easier turn because of the angle.

That's the 90-degree
The Pan Am flight crew now sees taxiway Charlie 1, which they're now just passing on their journey up the runway.

That's two.
The Pan Am pilot sees Charlie 2, which his plane is now passing in the fog. But from the transcript it's unclear what the flight crew subsequently considers the proper taxiway to turn on, Charlie 3 or Charlie 4. In the end, its decision to take the fourth rather than the third taxiway did contribute to the accident.

you are cleared
Air Traffic Control gives the KLM crew clearance, but, crucially, not to begin takeoff but only to fly a certain route after takeoff. Captain van Zanten on the KLM plane apparently mistakes the meaning of "clearance" here.

we're now at takeoff
This phrase used by the KLM copilot is nonstandard, as is the tower's vague "okay" after the captain starts powering up and says "Let's go." The copilot is apparently announcing that KLM is now taking off, yet the tower does not understand his comment this way, evidenced by its instruction moments later to stand by and wait for its clearance. In the aftermath of the disaster at Tenerife, officials implemented standard aviation phrasing worldwide to avoid similar confusion in the future.