einlegen
to insert
To put something into an opening intended for it or in a specific place. This often relates to technical devices or media.
Ich muss eine neue SIM-Karte in mein Handy einlegen.
I need to insert a new SIM card into my phone.

to insert / pickle
Loading your saved state.
to insert
To put something into an opening intended for it or in a specific place. This often relates to technical devices or media.
Ich muss eine neue SIM-Karte in mein Handy einlegen.
I need to insert a new SIM card into my phone.

'Einlegen' is a separable verb. The prefix 'ein-' goes to the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Er legt eine DVD in den Player ein.
He inserts a DVD into the player.
The verb is made up of 'ein' (in/into) and 'legen' (to lay/put). So the basic meaning is to put something into something else. Many of the meanings, including the figurative ones, can be derived from this.
Bitte lege die Wäsche in den Schrank.
Please put the laundry in the closet.
In the literal meaning, 'einlegen' (to insert) is often used for media or technical parts. Typical objects are: a CD, a DVD, a battery, or a SIM card.
Könntest du bitte neue Batterien in die Fernbedienung einlegen?
Could you please insert new batteries into the remote control?
In the context of food, 'einlegen' (to pickle/preserve) means that you preserve it. Typical combinations are 'Gurken einlegen' (to pickle cucumbers), 'Hering einlegen' (to pickle herring), or to pickle/preserve something 'in Öl' (in oil) or 'in Essig' (in vinegar).
Meine Großmutter hat früher immer Hering in eine Sahnesoße eingelegt.
My grandmother used to always preserve herring in a cream sauce.
In a formal or legal context, 'einlegen' (to file/lodge) is part of fixed expressions. You can say 'Widerspruch einlegen' (to file an objection), 'Berufung einlegen' (to file an appeal), or 'ein Veto einlegen' (to lodge a veto).
Der Anwalt riet seinem Mandanten, gegen das Urteil Widerspruch einzulegen.
The lawyer advised his client to file an objection against the judgment.
The expression 'eine Pause einlegen' (to take a break) is very common and simply means 'eine Pause machen' (to take a break). It is often used in the context of work, studying, or long trips.
Wir fahren schon drei Stunden, lass uns mal eine Kaffeepause einlegen.
We've been driving for three hours already; let's take a coffee break.
In connection with cars with manual transmission, the fixed expression 'einen Gang einlegen' (to engage a gear) is used. You can also 'den Rückwärtsgang einlegen' (engage reverse gear).
Du musst erst den Rückwärtsgang einlegen, bevor du ausparken kannst.
You have to engage reverse gear before you can back out of the parking space.
to insert / to begin / to use
'einsetzen' and 'einlegen' can both mean that you put something in a place intended for it. 'einsetzen' is used when something fits into an opening or position and usually sits there securely, for example a battery or a part in a device. 'einlegen' is often used when you put something into a device or in a place without it necessarily being firmly installed, for example paper in the printer. The words can be confused because both occur with devices. Pay attention: if something sits properly or is secured, 'einsetzen' often fits; if you place something inside, 'einlegen' often fits.

Vor dem Drucken legt Maria neues Papier in den Drucker ein.
Before printing, Maria loads new paper into the printer.
Tom setzt die Speicherkarte vorsichtig in die Kamera ein.
Tom carefully inserts the memory card into the camera.