SPF (Sun Protection Factor) refers to how well a sunscreen will protect you from UVB rays (think B for burning). For example, if your bare skin normally burns after 20 minutes in the sun, and you apply (and reapply) sunscreen correctly, then an SPF 15 will theoretically protect you from sunburn for 15 times longer -- about 5 hours.
This calculation does not apply to UVA rays, which are associated with long-term sun damage such as wrinkling. You’ll need a broad-spectrum sunscreen to protect against both types of rays.
No SPF can block 100% of UV rays, so be sure to also cover up and spend some time in the shade.