Summing up:

Contexts (tutorial/Contexts)

The basic idea is that the full name of any symbol is broken into two parts: a context and a short name. The full name is written as context`short, where the ` is the backquote or grave accent character (ASCII decimal code 96), called a "context mark" in the Wolfram Language.

• context`name or c1`c2`...`name a symbol in an explicitly specified context

• `name a symbol in the current context

• `context`name a symbol in a specific context relative to the current context

• name a symbol in the current context, or found on the context search path

Numbers

• Numbers entered in the form digits`p are taken to have precision p. ref/Precision

• Numbers entered in the form digits``a are taken to have accuracy a. ref/Accuracy

Templates (ref/TemplateSlot)

• In a string template given as a string, or in a file template, successive `` indicate TemplateSlot[i] for successive i.

• The form `name` for an alphanumeric string name not beginning with digits corresponds to TemplateSlot[1,"name"].

The character code is

In[1]:= ToCharacterCode["`"]
Out[1]= {96}

So, you may enter it by pressing the key combination: Alt+9+6.

Hi. There are many examples. The last is:

StringTemplate["a is ``, b is ``"][5555, 6666]

Can you look at your system settings and check what keyboard layout you are using?