You can convert strings to numbers using the function tonumber().  This takes a string argument and returns a number.

> print(tonumber("123") + 25)

148

 

> x = tonumber("123.456e5")

> print(x)

12345600

If a non-number string is passed, the tonumber() function returns "nil".

> x = tonumber("hello")

> print(x)

nil