MechWarrior 2: Mercenaries/Random missions
Random missions are always available, except when a crucial story campaign needs to be done. Each takes a month. The objectives are always in the three basic categories of recon (explore a target without destroying it), destroy (obliterate a target), and defend (ensure a target is not destroyed). Each has the secondary objective of eliminating all enemy resistance. You'll be told what the expected resistance will be (light, medium, or heavy), but this refers to enemy numbers rather the mech weight classes; once you've accepted the contract and boarded the contract you will be told the number of opponents and their actual classes.
Recon and Destroy are always very straightforward, but Defend are the more challenging; later in the game you'll have to fend off attacks on two different objectives. Having two lancemates is crucial.
Random missions almost always have salvage rights of poor. If you do a random mission during the month when a campaign will take place that campaign will be over by the time you return (unless it is a mandatory one, in which case it will still be there when you return). Since campaigns pay better and have more interesting missions you should always do them when they're available.
To practise the basic structure of the random missions without affecting your story progress, play some Instant Action matches.