This tool solves monoalphabetic substitution ciphers, also known as cryptograms. These are ciphers where each letter of the clear text is replaced by a corresponding letter of the cipher alphabet.
As an example here is an English cryptogram this tool can solve:
Rbo rpktigo vcrb bwucja wj kloj hcjd, km sktpqo, cq rbwr loklgo
vcgg cjqcqr kj skhcja wgkja wjd rpycja rk ltr rbcjaq cj cr.
-- Roppy Lpwrsborr
A Python implementation of this breaker is provided on GitLab.
If you want to break a polyalphabetic cipher instead try the Vigenère Solver.