In R udpipe package, if we code like:
library(udpipe)
x <- udpipe("The economy is weak but the outlook is bright. the property market will be booming next year", "english")
The result is:
doc_id paragraph_id sentence_id sentence start end term_id token_id token lemma upos
1 doc1 1 1 The economy is weak but the outlook is bright 1 3 1 1 The the DET
2 doc1 1 1 The economy is weak but the outlook is bright 5 11 2 2 economy economy NOUN
3 doc1 1 1 The economy is weak but the outlook is bright 13 14 3 3 is be AUX
4 doc1 1 1 The economy is weak but the outlook is bright 16 19 4 4 weak weak ADJ
5 doc1 1 1 The economy is weak but the outlook is bright 21 23 5 5 but but CCONJ
6 doc1 1 1 The economy is weak but the outlook is bright 25 27 6 6 the the DET
7 doc1 1 1 The economy is weak but the outlook is bright 29 35 7 7 outlook outlook NOUN
8 doc1 1 1 The economy is weak but the outlook is bright 37 38 8 8 is be AUX
9 doc1 1 1 The economy is weak but the outlook is bright 40 45 9 9 bright bright ADJ
xpos feats head_token_id dep_rel deps misc
1 DT Definite=Def|PronType=Art 2 det <NA> <NA>
2 NN Number=Sing 4 nsubj <NA> <NA>
3 VBZ Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin 4 cop <NA> <NA>
4 JJ Degree=Pos 0 root <NA> <NA>
5 CC <NA> 9 cc <NA> <NA>
6 DT Definite=Def|PronType=Art 7 det <NA> <NA>
7 NN Number=Sing 9 nsubj <NA> <NA>
8 VBZ Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin 9 cop <NA> <NA>
9 JJ Degree=Pos 4 conj <NA> SpacesAfter=\\n
I have a read through https://universaldependencies.org/ext-feat-index.html. But still, I cannot understand what feats means here?
These are morphological features of the words. Examples are gender, number, and case for nouns; person, number, aspect for verbs, etc.
This part of Universal Dependencies annotation is not universal at all. The page you referenced contains all morphological features that can appear in all languages that are in UD. Most of them are not applicable to most languages, some phenomena might appear multiple times under different names in different treebanks. To make the situation even trickier, some treebanks UDPipe ist trained do not contain the morphological features at all. UDPipe then of course only contains what it can learn from the treebanks.
UD contains six different treebanks for English and therefore there are six different models in UDPipe as well. There is an overview at the UD webpage that explains how the treebanks differ and also explains the morphological features that are used for English. The default for English is UD_English-EWT.