Question: What does this symbol in database mean ⊆?

for example for projection in relational algebra :

ΠLR

take a subset of the attributes of a relation R,
where L ⊆ R.

Answer: 
L ⊆ R means L is a subset of R.

For every element in L, the element exists in R.
L ⊆ R means L is a subset of R ('is contained in').

[L could also be equal to the whole of R].