what is BIO Tags for creating custom NER Named entity recognization?
It is easy. You need to tag a phrase using B (Begin), I (Interior), and E (End). For example, you want to tag "United States of America" as the name of a country. You will tag likes:

United(B_Country) States(I_Country) of(I_Country) America(E_Country)


In the same text if you find "Islamic Republic of Iran", you will tag likes:

Islamic(B_Country) Republic(I_Country) of(I_Country) Iran(E_Country)


Also, you will tag "United Kingdom" likes:

United(B_Country) Kingdom(E_Country)


Therefore, for every label that you have in the label set, you will have three labels in the tagging that is B_LabelName, I_LabelName, and E_LabelName. 

Notice that In some tagging systems, also uses from I_LabelName instead of E_LabelName.

Now, what is "O"? In some cases the all part of the phrase is not in the label but comming in the middle of the phrase and we need to say it is out ("O") of the label. For example, in the text analysis we want to take out "of" from country names. Hence we will tag "United States of America" like the following:

United(B_Country) States(I_Country) of(O_Country) America(E_Country/I_Country)