Used 295/25/21 Pirelli P Zero Nero 96Y 75% left

It's the indication of the maximum allowed speed.

Speed Rating	Maximum Speed (mph)
L	74
M	81
N	87
P	93
Q	100
R	106
S	112
T	118
U	124
H	130
V	149
Z	93 to 149+
W	104 to 167+
Y	115 to 186+

It's the indication of the maximum load allowed on the tire.

Load Index	Load Capacity (kg)	Load Index	Load Capacity (kg)	Load Index	Load Capacity (kg)	Load Index	Load Capacity (kg)	Load Index	Load Capacity (kg)
65	290	81	462	97	730	113	1150	129	1850
66	300	82	475	98	750	114	1180	130	1900
67	307	83	487	99	775	115	1215	131	1950
68	315	84	500	100	800	116	1250	132	2000
69	325	85	515	101	825	117	1285	133	2060
70	335	86	530	102	850	118	1320	134	2120
71	345	87	545	103	875	119	1360	135	2180
72	355	88	560	104	900	120	1400
73	365	89	580	105	925	121	1450
74	375	90	600	106	950	122	1500
75	387	91	615	107	975	123	1550
76	400	92	630	108	1000	124	1600
77	412	93	650	109	1030	125	1650
78	425	94	670	110	1060	126	1700
79	437	95	690	111	1090	127	1750
80	450	96	710	112	1120	128	1800