Multiple words separated by spaces are interpreted as a logical ‘and’:

Biden Obama

matches any text containing both ‘Biden’ and ‘Obama’ – in any order and position.

The ‘and’ can be made explicit:

Biden and Obama

is exactly the same as the ‘Biden Obama’ above.

The two other possible logical operators are ‘or’ and ‘and not’.

For example:

Biden or Obama

matches any text containing ‘Biden’ or containing ‘Obama’ or containing both; whereas:

Biden and not Obama

matches any text that contains ‘Biden’ but does not contain ‘Obama’.

The operator names ‘and’, ‘or’, ‘and not’ are case-sensitive: they must be lower-case.