An alphabet is any set of finite symbols such as a and b. For example, the alphabet Σ = {a, b} is an alphabet that contains the strings that can be built by combining a and b and the alphabet Σ = {0, 1} is the an alphabet that contains the strings that can be built by combining 0 and 1.

Symbols such as a and b put together to form something like bbaa are called strings.

bbaa is a string built from the symbols a and b

Alphabets and symbols can be used to create a language which is a set of strings over some fixed alphabet.

Given the language L = x ∈ {0, 1}* | x starts_with(x, 10):

S -> 1A
A -> 10B
B -> 10B | 1B | 0B | ε

We can try to match a few strings:

10 (Matches)
S -> 1A
A -> 10B
B -> ε

01 (Doesn't match)
S -> No match

101 (Matches)
S -> 1A
A -> 10B
B -> 1B
B -> ε

ε is the empty string and it is being used to stop the recursion.

Some of this comes from one of the books I’m reading at the moment: FORMAL LANGUAGE: A PRACTICAL INTRODUCTION