The 1206S4F2102T5E is a resistor manufactured by Royal Ohm. It is a surface mount resistor with a 1206 form factor, which describes its physical size. The first two digits, 12, refer to the length and width of the resistor in inches. The "S" following the four digits indicates that it is a thick film resistor with a power rating of 0402. The 4F indicates the resistance value, 4.0 ohms, with a tolerance of ±1%. The 2102 indicates the temperature coefficient of the resistor, which is 25 ppm/K, and the "T5" indicates that it has a 5% minimum tolerance. Finally, the "E" indicates that this is an E series resistor, which provides a slightly more precise tolerance than the standard 5% tolerance.