The SPF indicates the UVB protection. The amount of UVB radiation that is filtered out varies with different SPFs: SPF15 blocks 93% of UVB; SPF30 blocks 96.7% of UVB; SPF50 blocks 98.3% of UVB.
Moreover, the SPF50 has a higher UVA protection compared to SPF30. The star rating system is based on a ratio of UVA/UVB. Both are the maximum 5 stars, but as the UVB protection of the 50 is higher, the UVA is too.