The synthesis of virtually all proteins (mRNA->peptide) occurs in the cytoplasm. That’s where all ribosomes reside, after all. Ribosomes, which are mostly just rRNA (~2/3 rRNA + 1/3 protein*, by weight), are assembled in the nucleus but only do their stuff once they get to the cytoplasm.
For a protein to leave its original hometown of the cytosol and become a resident of the nucleus or, sayyyyyy, the endoplasmic reticulum, it needs to have a little string of amino acids which shout “I belong in the nucleus!” or “I belong in the endoplasmic reticulum!”
Proteins ultimately destined for the ER contain an unimaginatively named string of amino acids known as “signal sequence,” which, for the purposes of the Step 1, is always at the N-terminus. The signal sequence tells other cytosolic proteins, “Hey! Take me (and the rest of the peptide of which I am part) to the ER!”
In the absence of this signal, a protein will remain in its “default” home of the cytosol.
Here’s a nice schematic showing the flow of proteins from initial synthesis to final destinations:
*If you really want your mind blown, consider that even the protein subunits that make up that 1/3 of a ribosome are themselves initially synthesized in the cytosol; later, they are transported back into the nucleus via the nuclear pore.
I think this is describing a signal peptide (hydrophobic at N-terminus). Without signal peptide => can’t be transported into endoplasmic reticulum.