Experience points are cumulative, and characters level up when their experience reaches certain totals.
From the character advancement rules also found on PHB p. 15:
As your character goes on adventures and overcomes challenges, he or she gains experience, represented by experience points. A character who reaches a specified experience point total advances in capability. This advancement is called gaining a level.
Refer to the Character Advancement table to see which XP totals correspond to which character levels.
Suppose you start as a level 1 character with no XP. When you accumulate 300 XP, you become level 2. If you gain 600 more XP, your total XP becomes 900, and so you level up and reach level 3. And so on.