What does it mean to have a stroke above "a" in English poetry? what does thee mean, how to pronounce it

The way 'a is written may be to omit the letter before a. This is the same abbreviation we usually see.

For example: isn't=is not; couldn't= could not; 'tis[poetry]=it is

Old English thou=Modern English you (nominative case); Old English thee=Modern English you ( Accusative case);

Old English thy=Modern English your (possessive case)