Sessions - A session is the period time a particular user is actively engaged with your website. In general, if the number of sessions is going up over time, that is considered a good thing, as it means there is more traffic to your website.
Pageviews - Pageviews is the total number of pages viewed. Repeated views of a single page are counted. When a visitor hits the back button, a new pageview is recorded. When a visitor hits refresh, a new pageview is recorded. Every time a page is opened in the browser, regardless of whether it has been cached, a pageview is recorded. Increasing pageviews are also a reflection of increasing traffic, hence a good thing.
Users - Users who have initiated at least one session during the date range. A user is also a unique combination of a browser and a device. If I visit the same website in a different browser, or on a different device, although I am the same person, I am a different user. This is because Google Analytics puts a browser-specific cookie on each device used, and tracks users with that cookie. Increasing users is considered a good thing.
Mobile usage - percent of Sessions that were on a device other than a desktop computer (i.e., a tablet, phone, etc.).
New Sessions - An estimate of the percentage of first-time visits. Generally a good thing if you have many or increasing new users.
Pages / Session - (Average Page Depth) is the average number of pages viewed during a session. Repeated views of the same page are counted. Higher numbers are considered a good thing, but high numbers could also mean a user is mousing around on your website because they are not finding what they look for.
Avg. Session Duration - The average length of a session. Higher numbers can be positive as it is a measure of engagement, but see pages / session.
Bounce Rate - The percentage of single-page sessions in which there was no interaction with the page. See below for details.
More about Bounce Rate
A bounce is a single-page session on your site. In Analytics, a bounce is calculated specifically as a session that triggers only a single request to the Analytics server, such as when a user opens a single page on your site and then exits without triggering any other requests to the Analytics server during that session.
Bounce rate is single-page sessions divided by all sessions, or the percentage of all sessions on your site in which users viewed only a single page and triggered only a single request to the Analytics server.
These single-page sessions have a session duration of 0 seconds since there are no subsequent hits after the first one that would let Analytics calculate the length of the session.
Is a high bounce rate a bad thing? It depends.
If the success of your site depends on users viewing more than one page, then, yes, a high bounce rate is bad. For example, if your home page is the gateway to the rest of your site (e.g., news articles, product pages, your checkout process) and a high percentage of users are viewing only your home page, then you don’t want a high bounce rate.
On the other hand, if you have a single-page site like a blog, or offer other types of content for which single-page sessions are expected, then a high bounce rate is perfectly normal.