'This/That is' + one thing
Use 'This is' when you talk about one thing that is close to you. Use 'That is' when you talk about one thing that is far from you.
For instance, if you see an airplane in the sky, you do not say 'This is an airplane'; you say 'That is an airplane.'
'These/Those are' + many things
Use 'These are' when you talk about more than one thing that is close to you. Use 'Those are' when you talk about more than one thing that is far from you.
"What is this/that?" and "What are these/those?" are useful phrases to use when you want to know the name of something in English. Use them with English-speaking friends and quickly learn new vocabulary.