The corona, the outer atmosphere of the Sun, is a multi-million degree plasma, nearly three orders magnitude hotter than the visible surface. The exact mechanism by which the corona is heated is still the subject of debate, but possibilities include magnetic reconnection and magnetohydrodynamic waves. Studying the thin boundary layer connecting the cooler chromosphere to the hotter corona, named the TR, is an important step toward understanding mass and energy transport from the chromosphere to the corona. Thus, spectral emissions from the cool (< 1 MK) loop-like structures in this region are in need of extensive study and analysis. Because observations lack sufficient spatial resolution, this type of structure was called the “unresolved fine structure”, which is now considered resolved by the Interface Region Imaging Spectrograph (IRIS). In the active TR of the Sun, IRIS has observed loop-like structures with intermittent brightenings which are thought to originate from impulsive heating. In this thesis, the author present evidence of magnetic field line braiding and reconnection mediated brightenings of TR loops using IRIS slit-jaw images and spectral data, complemented by the EUV channels of the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). The set of observables used to characterize the brightenings consists of diagnostics of temperature, density, line broadening, and Doppler-shift on a pixel-by-pixel basis. The characterization scheme is extended by accumulating time dependent differential emission measure (DEM) distributions to define the nature of the spatial heating profile and frequency. A field-aligned hydrodynamic simulation and a forward modeling code, designed to generate synthetic observations from numerical experiments for comparison with real data, are employed. Non-equilibrium ionization is included in the computation of synthetic spectra. In addition, the relatively high-density TR plasma requires the inclusion of density-dependent dielectronic recombination rates to calculate the ion populations and the emission line intensities. We show that the observations and the numerical experiments are consistent with reconnection mediated impulsive heating at the braiding sites of multi-stranded TR loops. The combination of observation and numerical analysis will provide the building blocks of time-dependent 3D models of these loops and their contribution to active region emission which will, in turn, help us to understand the energy balance of these structures and may shed light on the long standing coronal heating problem: “Why is the Sun’s corona so much hotter than the surface?”.