sicher
sure
Being convinced of something; without doubt.
Ich bin mir sicher, dass ich die Tür abgeschlossen habe.
I'm sure that I locked the door.

sure / safe
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sure
Being convinced of something; without doubt.
Ich bin mir sicher, dass ich die Tür abgeschlossen habe.
I'm sure that I locked the door.

The adjective 'sicher' (sure/safe) is the basis for the noun 'die Sicherheit' (certainty/safety/security). This noun can mean both certainty and protection from danger, just like the adjective.
Die Sicherheit der Gäste hat für uns oberste Priorität.
The safety of the guests is our top priority.
When you want to express that you are convinced of something, you use the structure 'sich (Dativ) sicher sein' (to be sure). The reflexive pronoun in the dative is very important here.
Bist du dir sicher, dass du den Herd ausgemacht hast?
Are you sure that you turned off the stove?
To express what someone or something is protected from, the preposition 'vor' (from) is used with the dative.
Hier im Keller sind wir vor dem Gewitter sicher.
Here in the basement, we are safe from the thunderstorm.
As an adverb, 'sicher' (surely/certainly) expresses a high probability. It usually appears in the middle of the sentence and does not change the word order.
Keine Sorge, der Bus kommt sicher in ein paar Minuten.
Don't worry, the bus will surely come in a few minutes.
maybe
'vielleicht' and 'sicher' are opposites when it comes to certainty. With 'vielleicht', you say that something is possible but not certain. With 'sicher', you say that you have no doubt or that something is very certain. So 'vielleicht' fits when there is uncertainty, while 'sicher' fits when there is clarity or strong conviction.

Nach dem Anruf bin ich sicher, dass der Termin morgen stattfindet.
After the phone call, I am sure that the appointment will take place tomorrow.
Vielleicht kommt der Bus heute ein paar Minuten später.
Maybe the bus will come a few minutes later today.