Includes a textfile into the output. The contents of the file is not parsed in any way, so you cannot use skin/templatevars or use PHP code (see parsedinclude and phpinclude if you want parsed includes)
IncludeMode and IncludePrefixall
<%include(filename.txt)%> <%include(/home/user/myself/filename.txt)%> <%include(http://mydomain.com/filename.html)%>