Open a community and you land on Feed. Two things run down it, newest first: updates and news from the community’s administrators — a call for proposals opening, a deadline moving, a new issue out — and posts from members talking to one another. The box at the top is where you join in: share something you have written, pass on news, or ask the community for a response.
Following lets you read the feed — announcements included. Posting to it is for members

Author, affiliation, time, the post, anything attached, and the four actions. The affiliation is the useful one — it places someone you have never met faster than their name does.
The four actions are the same everywhere on the platform. Your feed has the full key.

There are two feeds on the platform, and they are easy to confuse. This one belongs to one community. Your feed, on Home, merges every community you follow or belong to — and you can write to either.
You can always post to your own feed. Posting to a community’s feed is the part that needs membership.

A post you write to a community is submitted rather than published outright. It goes through a light review by the community before it appears in the feed.
So a post may not show up the moment you send it. That is the review, not a fault.
Once it clears review, the post is listed in three places: at the top of that community’s feed, in the Home feed of everyone who follows or belongs to it, and under Manage my posts, where you can edit or remove it later.

Posting to a community’s feed is for its members. Following lets you read it. You can still post to your own feed at any time — that never depends on a membership.
Who it is addressed to. A post written in a community goes to that community and its followers, and needs you to be a member. A post to your own feed is yours — always available, and reaching the people who follow you.
Lightbulb to like, bubble to comment, bookmark to save, and share. Your feed carries the full key with an example of each.
Community posts go through a light review before they show in the feed. Give it a little time; if it stays missing, raise a support request from the community.
The community’s administrators. Their updates — calls for proposals, deadlines, new issues — arrive in the feed alongside members’ posts, so the feed is worth reading even in a quiet week.
Manage my posts, under your profile picture. Everything you have written — personal posts and community posts alike — in one list.
Feeds follow their communities. A small interest group may go weeks between posts while a large academic society moves daily — the member count on the community card is a fair guide.