In a competitive golf tournament, golfers are expected to play at a reasonable pace. What constitutes a reasonable pace, though, can vary from tournament to tournament.
Presenting organizations -- like the USGA, R&A, PGA of America, LPGA and PGA Tour -- all have different timing rules to which players have to adhere, but they all have a similar set of enforcement standards for when players fall behind and start playing slower than expected pace.
Typically, when a group or player starts falling behind the pace of play, someone involved with the tournament, namely a rules official or ranger, will approach the group and explain to them that they've fallen behind the pace and need to go faster. If that interaction doesn't seem to change the pace of play, then things start to escalate. The next step is to put a group or a player "on the clock."
What does it mean to be put "on the clock" in golf?
In golf, being put "on the clock" means that a player or each player in a group is being timed on each and every shot to make sure they are taking no more than the allotted amount of time to hit each shot. Golfers may need to take longer on a hole because they're not scoring well, but they are expected to play the ball in a limited amount of time when it's their turn.
On the PGA Tour, pros have either 40 or 60 seconds to hit the ball on their turn, depending on the situation.
The USGA has a concept of time par, which is a pace-of-play standard for groups that outlines how much time each group should take to play segments of the course. The groups check in with a monitor at different points in the round to make sure they're on schedule.
Either way, golfers have to be notified if they've fallen behind and that they could be subject to a pace-of-play penalty. At that point, they're on the clock. If they continue to remain behind pace, it's at that point that a player or a whole group individually can face stroke penalties for slow play. That almost never happens. Typically, being "on the clock" is enough to scare players into catching up to the pace.