TypoScript is line based. Each line normally contains three parts:

[Identifier] [Operator] [Value]

In this example we have the identifier myIdentifier with the sub identifier mySubIdentifier, the assignment operator = and the value myValue.

Extension examples, file Configuration/TypoScript/Syntax/General/setup.typoscript
myIdentifier.mySubIdentifier = myValue

The identifier path (in above example myIdentifier.mySubIdentifier) is a dotted path of single identifiers, and the first block of non-whitespace characters on a line until an operator, a curly open brace, or a whitespace. The dot (.) is used to separate single identifiers, creating a hierarchy.

When a dot is part of a single identifier name (this may, for instance, sometimes happen when configuring FlexForm details), it must be quoted with a backlash. The example below results in the identifier myIdentifier with the sub identifier my.identifier.with.dots having the assigned value myValue:

Extension examples, file Configuration/TypoScript/Syntax/Escaping/setup.typoscript\.identifier\.with\.dots = myValue