I wasn't even sure what an SPF rating means but REI seems to have a good page on UPF (the equivalent for clothing): Sun Protection Clothing Basics
Basically, a UPF rating of 50 indicates the fabric of a garment will allow only 1/50th (roughly 2%) of available UV radiation to pass through it. A garment rated UPF 25 permits roughly 4% (1/25th) UV transmission.
UPF covers both UVA and UVB (not sure if they "average" the two or how exactly the protection rating is combined).
On t-shirts they write:
So where does a white cotton T-shirt rate? Many Web sites estimate it falls between UPF 5 and UPF 8, meaning it could allow as much as 20% (one-fifth) of available UV radiation to pass through.
While no doubt true in some cases, many newer T-shirts are treated with "optical brightening agents." These OBAs, appearance-enhancers for white fabrics, also boost disruption of UV radiation. Most common household detergents also include OBAs, so repeated launderings will increase the fabric's accumulation of brighteners and thus increase its UV-protective ability. Some experts estimate that such shirts may offer a UPF of nearly 15.
So at the low end a t-shirt could block 80% of UV light but could be as high as 95%.