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There are a variety of roles the lock plays. The first is your general workhorse. Behind the hooker (dummy half) the lock tends to make the most tackles and generally take a high number of runs because they are usually the teams fittest forward so when everyone else is exhausted they can keep going. However specific roles vary. Lock forwards are capable of playing in a variety of ways. The first is like a third prop (much like Paul Gallen), he makes a lot of tackles and runs the ball a lot, he spends a lot of time in the "centre of the action" doing all the "dirty work" kinda stuff. The second is more like a third second-rower, the difference is almost down to their "finesse" (Feleti Mateo is a good example) where he can play around the ball a lot, almost like a half, but still takes a lot of runs. The final one is more like a second hooker, where they make a lot more tackles and slot well into the hooker roll if the hooker is injured. (Elijah Taylor for example). The difference between the three is the areas of the game where they exert most of their talent, the first into ball running, the second into finesse and general game-play, the third into defence. These of course are not all the ways a lock can play, it is just a few of the most common examples. But to actually answer the question of what IS the lock forwards ROLE, the best answer i can think of is a general workhorse who can fill in in pretty much any position (although wing and fullback are a stretch) and still contribute some impact from their role