Some versions* of
find require that you provide a path argument which is a directory from which to start searching. Dot
. simply represents the current directory is is usually where you want to search.
You could replace this with any path that you want to be the base of the search. In some versions of find this can be left because the current directory is implied if no path argument is present.
You can run
man find in your shell for details about the arguments. For example the usage synopsis for mine indicates that the path argument is optional (inside square brackest
find [-H] [-L] [-P] [-D debugopts] [-Olevel] [path...] [expression]
If you ran my
find with no arguments at all all files and directories starting from the current folder would be returned. Your example simply expressly states that the search should start from
. and includes the expression
-name foo.txt as one of the search filters.
* Notably all the BSD variants and anything sticking strictly to the POSIX standard. GNU find allows it to be optional.