die Sicherheit
safety
The state in which one is protected from dangers or risks.
Die Sicherheit der Fahrgäste hat für uns oberste Priorität.
The safety of the passengers is our top priority.

safety / certainty / security
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safety
The state in which one is protected from dangers or risks.
Die Sicherheit der Fahrgäste hat für uns oberste Priorität.
The safety of the passengers is our top priority.

'Sicherheit' (safety/certainty/security) is the noun corresponding to the adjective 'sicher' (safe/sure). The ending '-heit' is often used to turn an adjective into an abstract noun that describes a state or quality.
Ich bin nicht sicher, ob das die richtige Entscheidung ist.
I'm not sure whether that is the right decision.
In the context of protection from danger, 'Sicherheit' (safety) is often used with certain verbs. Typical expressions are 'jemanden in Sicherheit bringen' (to get someone to safety), 'für Sicherheit sorgen' (to ensure safety), or 'die Sicherheit gewährleisten' (to guarantee safety).
Die Feuerwehr brachte die Bewohner des Hauses in Sicherheit.
The fire department got the residents of the house to safety.
When it is about certainty, 'Sicherheit' (certainty) often appears in phrases such as 'mit Sicherheit sagen' (to say with certainty), 'absolute Sicherheit haben' (to have absolute certainty), or 'keine Sicherheit geben' (to give no certainty/guarantee).
Ich kann dir leider keine hundertprozentige Sicherheit geben, dass der Plan funktioniert.
Unfortunately, I can't give you one hundred percent certainty that the plan will work.
This meaning occurs almost only in a financial or legal context. Signal words are verbs such as 'hinterlegen' (to deposit) or 'leisten' (to provide), as well as nouns such as 'Kredit' (loan), 'Miete' (rent), or 'Kaution' (deposit).
Für den Kredit verlangt die Bank eine Sicherheit.
For the loan, the bank requires collateral.
In this meaning, 'Sicherheit' (security) is used in the singular but refers to a group of people. You can recognize it when the word, as the subject, performs an action, e.g., 'die Sicherheit kontrolliert' (security checks) or 'die Sicherheit greift ein' (security intervenes).
Die Sicherheit hat den Mann aus dem Stadion entfernt.
Security removed the man from the stadium.
The fixed expression 'mit Sicherheit' (certainly/with certainty) functions like an adverb expressing a high degree of certainty. It can stand at the beginning of a sentence (then the verb follows) or in the middle of the sentence.
Mit Sicherheit wird es morgen regnen.
It will certainly rain tomorrow.
danger
'die Sicherheit' and 'die Gefahr' form a clear contrast. 'Sicherheit' means a state in which you are protected and there is no major risk. 'Gefahr' means a situation in which harm can happen. The words often fit into the same subject areas, for example traffic, work, health, or housing. When the danger is over, people talk about 'Sicherheit' again or say that someone is 'außer Gefahr' (out of danger).

Nach dem Einbau der neuen Tür fühlt sich die Familie zu Hause in größerer Sicherheit.
After the new door was installed, the family feels safer at home.
Wegen des starken Sturms besteht auf der Brücke große Gefahr.
Because of the strong storm, there is great danger on the bridge.