The fortis-lenis distinction is present in all languages and essentially differentiates between realizations of the same sound. So, for example in Swiss German, P, T, K are unaspirated in the initial position (Thomas is pronounced “Domas”), whereas B, D, G are always voiceless. In written Swiss German, this feature can be seen in doubled consonants: compare “Glunggi” with “träge”.
English makes this same distinction with the following pairs of sounds:
p-b t-d k-g f-v ɵ-ð s-z tʃ-ʤ ʃ-ʒ
(The funny IPA symbols usually correspond with the letters th-th (think v. that); tch – dg; and sh – g.)
The first is the fortis, the second the lenis. So “pub” is pronounced with a harsh initial consonant and a soft end one. This can be just the opposite of Swiss German pronunciation: try German “Tag” with English “tag”.
For a mouthful, try pronouncing these: chop – job, pack – bag, French – fringe, love – laugh, chess – cheese – jazz.