The words primer, primmer sound the same but have different meanings and spellings. Why do primer, primmer sound the same even though they are completely different words?
The answer is simple: primer, primmer are homophones of the English language.
One who or that which primes.
A small powder-horn containing fine powder used for priming.
Any one of many cerambycid beetles whose larvæ bore into the twigs of various trees, weakening them and causing them to fall off as if pruned.
First; original; primary.
<xref>comparative</xref> form of <xref>prim</xref>: more <xref>prim</xref>
Definitions from The Century Dictionary., from Wiktionary, Creative Commons Attribution/Share-Alike License. and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").