In our physical world, by far, the most abundance free charge in the universe is the electron, which has a negative charge. what do we mean, then, when we say that the surface of an object is positively charged?

here the answer is obvious.we know that the charge of electron is negative and all the substance contains electrons.but there is also another particle called proton which is also a fundamental particle.

As per the question,the surface of an object is positive.it means  that some amount of electrons have been emitted by the object.

when the electron emission takes place from the surface of the object,the number of electrons is decreased .as the atom contains same number of protons and electrons making the whole atom neutral ,so, when electron is lost,there will more number of proton as compared to electrons.The surface gains a positive charge.

The surface becomes positive as generally the electrons present on the surface are less bound to the nucleus and can be easily escaped from the surface barrier.