What the hot Big Bang actually says
The hot Big Bang is the well-tested framework in which the early universe was hotter, denser and more nearly homogeneous than it is today. As the scale factor grew, the plasma cooled, nuclei formed, atoms formed, the cosmic microwave background was released, and gravity later amplified small density variations into galaxies and the cosmic web. This framework does not by itself tell us what dynamical process produced the initial hot state.
Why inflation is introduced
Inflation is a period of accelerated expansion. In the simplest models a scalar field has energy dominated by a potential V(φ), giving an effective negative pressure. Rapid expansion can stretch a small region to enormous size, dilute curvature and unwanted relics, and — crucially — stretch quantum fluctuations to astronomical scales where they become seeds for later structure.
How reheating makes a hot universe
Inflation cannot simply continue forever in our local history. Its energy must be transferred into ordinary excitations. During reheating the inflating field loses energy to other degrees of freedom; radiation grows until the universe becomes radiation dominated. From that point the standard hot-Big-Bang thermal history takes over.
What can be tested
Inflationary models predict a primordial scalar spectrum, possible tensor perturbations, spectral tilt, non-Gaussianity and model-dependent features. Reheating affects the mapping between model parameters and the number of e-folds. The CMB and large-scale structure therefore constrain the inflationary history indirectly, even though we do not observe inflation itself as a photograph of the past.
What remains open
Inflation plus reheating is a powerful bridge, but it does not uniquely answer why an inflating state existed, whether there was an earlier phase, or whether the theory is past complete. Those are separate origin questions and motivate bounce models, quantum cosmology and other alternatives.