BattleTech – Mercenaries isn't a distinct rules set, strictly speaking, as you use the rules from BattleTech for your games. Mercenaries brings rules for a kind of campaign setting. You create your own mercenary unit, starting with ~2 BattleMechs. There are rules for accepting contracts, what the contract pays for concerning upkeep cost, what salvage rights you get, etc. The campaign is measured in months, cost arises each month.
Then you play missions, as defined by the contract. First, you need to travel to the planet your contract is based at, which might or might not already incur cost (some contracts pay for that) and is given in months. Then you have to fight your missions, usually one per month. When your 'Mechs get damaged, you need to repair them. You need to restock ammunition. All of which costs money. Lost 'Mechs cannot simply be replaced, you need to check the market where only a limited number of 'Mechs is available at any one time.
So Mercenaries provides a framework for your basic rule BattleTech games.
Well, unless you come to support units like tanks, helicopters, artillery support, etc. In the base game, a tank is like a 'Mech – you have a sheet (different to a 'Mech's, but of the same complexity/level of detail), the combat is as detailed, etc. The mercenaries use a different system for support assets (when my understanding is correct based on the rules in BattleTech: Alpha Strike), which keeps the game 'Mech focussed, allows using the support units (which is essential, as it balances the difference in 'Mech battle value between opposing forces), but not taking up much time on the support units.
Then there are the Hot Spots. For Mercenaries, Catalyst is publishing Hot Spot books, that are focussing on a specific setting. The missions are related to the fluff of that specific setting, as are the market tables for getting 'Mechs, for example. So Mercenaries has great re-playability, as each Hot Spot changes the frame your are playing in.