The term Mid is a catch-all term for those who are on both the aromantic spectrum and the asexual spectrum while not being strictly aromantic and strictly asexual.
The term comes from the word middle, as in middleground or in the middle, describing how those who identify with it feel in between being aromantic/asexual and alloromantic/allosexual. It's the suffix used for the three identities under its umbrella:
Aromid, those who are strictly aromantic but not strictly asexual, while still being on the ace spectrum
Amid, those who are on the asexual and aromantic spectrums, but are not strictly aro or strictly ace
Acemid, those who are strictly asexual but not strictly aromantic, while still being on the aro spectrum
The flags are all amalgations of the aroace flag by aroaesflags, who describes the flag's meaning here. In place of the orange (community and union) and yellow (forms of love outside of sex and romance) are shades of green for aromantism in the aromid flag, and purple for asexuality in the acemid flag, the multiple shades representing community and diversity among aromantics and asexuals.
The amid flag uses shades of grey in place of yellow and orange. The term grey is used for those who find themselves in between a lack of and presence of attraction (greysexual, greyromantic). They also come in multiple shades to represent community and diversity.
As the blues represent a combination of asexuality and aromantism, those remain present in the mid flags, as does the white stripe for unity.
The flags in order from left to right are the acemid flag, the amid flag, and the aromid flag.
The official blog by the coin termer can be found here. Questions can be directed towards them.