After This
From Now On, After, Since
noun
From after, just means after this or from now on.
いご
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
「以後、このようなことがないように、気をつけるんだぞ。」「はい、先生。以後、もっと気をつけます。」
"After this you need to be more careful so this doesn't happen again." "Yes, teacher. I'll be more careful from now on."
10月29日のトーフグの日以後、ビエトは行方をくらましているんだ。
Viet hasn't been heard of since October 29—Tofugu Day.
それ以後、その作家は酒を一滴も飲まなくなった。
After that, the writer stopped drinking alcohol, not even a single drop.