Verb (type 1): to undress, strip, take off.
See riisua conjugated here.
Riisua appears in our example as riisuthan. Riisuthan is constructed from riisut, the 2nd person singular form of riisua, with the particle -han (linguistically an enclitic) appended. The particle –han can be appended to a verb to indicate a wish or to politely ask someone to do something instead of using the imperative. (The front vowel equivalent is –hän.) See additional useful info about this particle here.
A good translation of riisuthan into English might be, ”Undress, will you?” or ”If you would undress…” (In our example, ”take off” is the closer meaning.)
Let’s translate the whole sign, ”Riisuthan ystävällisesti kenkäsi,” or ”If you would kindly take off your shoes.”
Kenkäsi means ”your shoes” in this phrase. It is constructed from the (strong) vowel stem of kenkä (”shoe”) with the suffix–si, a shortened form of the possessive pronoun sinun (”you”), appended. Note that there’s no distinction between singular and plural possession in this case. Kenkäsi here can mean both ”your shoe” and ”your shoes,” but the correct meaning is understood.