The attribution attached to a line of speech - 'she said', 'he asked' - marking who is talking.
A dialogue tag is the small attribution that tells the reader who spoke: 'said Mara', 'he muttered'. In prose fiction the workhorse is 'said', which the eye skips almost invisibly, doing its job of assigning the line without competing with it.
The common overcorrection is dressing every tag up - expostulated, opined, hissed - or loading each with an adverb, which signals distrust of the dialogue itself. Strong scenes lean on plain tags and action beats, letting word choice and rhythm carry the tone the fancy verb was reaching for.
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