heißen
to be called
To have a particular name.
Wie heißt du?
What is your name?

to be called / to mean
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to be called
To have a particular name.
Wie heißt du?
What is your name?

When you say what someone or something is called, the name is in the nominative. It is a simple structure: subject + heißen (to be called) + name.
Sein bester Freund heißt Thomas.
His best friend is called Thomas.
The perfect tense of 'heißen' (to be called) is formed with 'haben' and the form 'geheißen'. You use it to talk about the former name of something or someone.
Die Straße hat früher anders geheißen.
The street used to have a different name.
The question 'Was soll das heißen?' (What is that supposed to mean?) is a very common fixed expression. You use it to express dissatisfaction or surprise about a statement or situation.
Du gehst ohne mich? Was soll das heißen?
You're going without me? What is that supposed to mean?
The expression 'es heißt' (it is said) often introduces a subordinate clause with 'dass' (that). In this subordinate clause, the conjugated verb comes at the end.
Es heißt, dass die Firma bald neue Mitarbeiter einstellt.
It is said that the company will soon be hiring new employees.
The meaning 'befehlen' (to command) or 'anweisen' (to instruct) is very rare today and sounds formal or old-fashioned. You will almost only find it in literature.
Der König hieß den Diener, ihm Wein zu bringen.
The king ordered the servant to bring him wine.
In the expression 'jemanden willkommen heißen' (to welcome someone), 'willkommen' behaves like a separable prefix. In a main clause, it comes at the end.
Der Bürgermeister hieß die neuen Bürger in der Stadt willkommen.
The mayor welcomed the new citizens to the city.
to say
'sagen' and 'heißen' can both have to do with meaning. 'heißen' is used very often when asking about the meaning of a word, sentence, or sign: 'Was heißt das?' 'sagen' fits better when something conveys a statement, sign, or piece of information: 'Das sagt viel über ihn.' Learners confuse the words because both can sometimes be replaced with 'bedeuten'. As a simple guide: with names and word meanings, 'heißen' is usually more natural. With a message or statement, 'sagen' often fits.

Was heißt dieses Wort auf Deutsch?
What does this word mean in German?
Sein Lächeln sagt mir, dass er zufrieden ist.
His smile tells me that he is satisfied.