8 × 50: how many and how far
The first number is the number of repeats and the second is the distance of each repeat. 8 × 50 m therefore means eight 50 m swims, for 400 m total.
The same pattern applies to 4 × 100, 12 × 25, or 3 × 200. The total is not always written because workout notation focuses first on how the distance is divided.
Leave every 1:10 versus 20 s rest
Leave every 1:10 means each repeat begins 1 minute 10 seconds after the previous one. Swim the 50 in 50 seconds and you get 20 seconds rest; swim it in 55 and you get 15.
20 s rest means you always wait 20 seconds after finishing. Your next departure therefore depends on your swim time. The two instructions are not interchangeable.
IM: individual medley
IM means individual medley. The stroke order is butterfly, backstroke, breaststroke, then freestyle. In a 100 m IM in a 25 m pool, you swim 25 m of each stroke in that order.
French Canadian workouts often use QNI for quatre nages individuelles. Be careful with medley relays: the competition order is different because backstroke comes first.
Information
Swim, drill, kick, pull
Swim usually means the normal full stroke. A drill isolates or exaggerates part of the technique. Kick focuses primarily on the legs; pull reduces their contribution to emphasize the upper body. Equipment instructions tell you how the block should be done.
Easy, moderate, strong, fast
These words describe an effort intention rather than one universal time. Easy should leave you in control. Moderate requires work but is sustainable. Strong is demanding and needs more focus. Fast aims for real speed, often over shorter repeats with enough recovery to preserve that speed.
A workout may also use zones or a more precise effort scale. In that case, follow the definition provided with the workout rather than inventing your own.
Build, descend, negative split
Build means increase speed across a repeat or group of repeats. Descend usually means each repeat gets faster: 46, 44, 42, then 40 seconds, for example.
A negative split means the second part of a swim is faster than the first. It does not mean drifting through the first half and sprinting the finish; the idea is to manage effort so you can close faster.
Choice does not automatically mean freestyle
Choice lets you select the stroke, usually within the purpose and context of the set. Freestyle is common because it is comfortable for many swimmers, but another stroke can be appropriate if it respects the instruction.
And if a term is still unclear, ask. The vocabulary exists to make workouts shorter on paper, not to create a barrier to entry. After a few practices, 8 × 50 IM leaving every 1:10 becomes an ordinary sentence.