[Tutor] what %s=%s means?

[=?GB2312?B?uqvP3Ma9?=]

I realy new to python.I try on learning it by code examples.The
following one from "Dive into python":

def buildConnectionString(params):
    """Build a connection string from a dictionary of parameters.

    Returns string."""
    return ";".join(["%s=%s" % (k, v) for k, v in params.items()])

if __name__ == "__main__":
    myParams = {"server":"mpilgrim", \
                "database":"master", \
                "uid":"sa", \
                "pwd":"secret" \
                }
    print buildConnectionString(myParams)

Whant ";"means
%s=%s?
join?
>>>help(items) no matched.
Where can i find stuff helpful?