R&D Lock is when the runner manages to arrange a Board Position where they are able to run R&D often enough and to access enough cards, so that it is difficult or impossible for the Corp to draw any Agendas.
Typically this is achieved by running once a turn to access 2-3 cards with the help of R&D Interfaces. Although it can be achieved in other ways such as a Medium run every other turn, or an Indexing every 3rd turn to see 5 cards ahead each a time (perhaps with the help of Same Old Thing to recur them).
The Corp can break R&D Lock by installing and rezzing more ice on R&D that either stops the runner or makes it too expensive to keep running so regularly. Or in a pinch, especially when looking for just one more Agenda to win, by using a card that triggers a deck shuffle then drawing a few times immediately.