The /Ђ/ and /Ћ/ characters are new. The lower-case versions are /ѣ/ and /ћ/.
These characters are used to replace the two character combo /th/ (aka 'digraph') used in current English.
The /t/ and /b/ or the /t/ and /h/ are combined into a single letter.
These consonant sounds are known as the voiceless and voiced dental fricatives.
The voiceless version combines 'T' & 'b'.
The voiced version combines 'T' & 'h'.
Remember 'tooth' vs 'smooth'. You hum through the /th/ sound in 'smooth'.