“Watch and pray, lest you enter into temptation. The spirit is willing, but the flesh is weak” (Mark 14:38).
Jesus is in the Garden of Gethsemane with his three closest apostles when he gives them this sobering advice while they are sleeping. Peter has just vowed his undying love for the Lord, but already he has forgotten and fallen asleep.
“Watch” means “stay awake” or “look out”; and “pray,” of course, means they should be vigilant to keep God close in order to stay strong enough to overcome temptation.
“Temptation” itself is not wrong; but “entering in” to temptation means we have, at least, allowed ourselves to stay around it. Such an action puts us in a position to “give in,” at which point we commit the sin suggested. Like us, these three wanted to stay faithful to the Lord; Jesus knew that about them. But, again, like us, they were human; and their flesh might allow them to give in to sin at a weak moment.
So, the point of the teaching is to guard ourselves by being careful about where we go, whom we associate with, and what we allow to enter into our minds, thus potentially leading us away from the Lord.