Your posting reads as if you are quoting an article, possibly a website. If so, please provide a link to that document.

<Edit> After some searching, I realised that you are describing the Account Central page in Thunderbird which offers links to various features, including a search. That same search can be more efficiently accessed using the keystroke ctrl+shift+f. Oddly, what you describe sounds like the closely related Filter dialogue, because you include the actions (e.g. move, copy, delete) that can be taken by a filter.

The top half lets you set up a search. It defaults to "Subject" "Contains" and a box for you to type in what words in subject lines you want to search for.

So,

Subject Contains anniversary

will find all messages in the current folder (and optionally its subfolders) where the word anniversary appears in the subject. The filters don't care about capitalisation, so it will find Anniversary as well.

You can change any of these. So you could search for messages from Anne.

From Contains Anne

You can use the name ("Anne") or her email address here. Note that the list of "verbs" (Contains, Is, Begins with, Ends with) varies according to which part of the message you are searching for. This rule will find all messages from anyone with Anne as part of their name. To make it more precise, include their full name (as it appears in email messages), or better still, their email address.

You can extend this to search for both "messages about anniversaries" or "messages from Anne". This is where the '+' comes in. You set up two rules:

Subject Contains anniversary

and then use + to add another rule:

From Contains Anne

Now, do you want messages that either mention anniversary or are from Anne, or do you want only messages from Anne about an anniversary?

The first is an 'OR', the second is an 'AND'. You choose between OR and AND at the top.

Match all of the following means that all your rules must be satisfied, so represents an AND.

Match any of the following means that you only require at least one of your rules to be satisfied for a message to be included. We'd call this an OR.

So,

"Match any of the following"

From Contains Dave From Contains Anne

will find all the message from either Anne or Dave. Note that, in general, "Match all the following" would fail here because usually a message cannot simultaneously come from two different email addresses, unless Anne and Dave are one of those unimaginative couples who share an email address, as in: "anneanddave@example.com". (I hate these; how do I communicate privately with Anne to discuss Dave's surprise birthday party?)

In a search, the bottom half is where a summary of matching messages is shown.

In a filter, it is where you specify what to do with the matching messages. Filters are often used to automatically delete unwanted messages, or to move messages matching certain criteria to a designated folder, or to tag them for easy identification.

These searches and filters are a bit limited, If you want to find messages about the anniversary from either Dave or Anne, there is no straightforward way to combine the OR ("Dave OR Anne") with the limitation that the anniversary must also be mentioned.

A big thank you to the Forum contributors to the solution of my problem. It will help, the computer illiterate that I am, and hopefully it will help someone else along the way. THANK YOU!!!