Convertible Note A convertible note is a vehicle for raising funding. It's a promissory note, but instead of being repaid as cash, the debt is converted into equity during your next priced round.
Valuation Cap - Uncapped The valuation cap puts a ceiling on the valuation at which the note converts to equity. If you raise at a $6mm valuation and an investor holds a note with a $3mm valuation cap, their equity is allocated as if the valuation were at $3mm.
Uncapped means that there is no valuation cap. The note will convert at the $6mm valuation.
MFN (Most Favored Nation) clause A most favored nation clause guarantees that investor the best terms - if you offer a second investor a better deal, the first investor gets upgraded to that deal as well.
All together: An uncapped convertible note with an MFN clause Putting it together, these notes are the most flexible terms you can possibly have.
Having no cap could mean that the investor could get fewer shares when the note converts (compared to the number of shares that the investor could get with a cap).
I've never seen a convertible note without a cap.